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Subjects

Abstract

Accreting supermassive black holes (SMBHs) can exhibit variable emission across the electromagnetic spectrum and over a broad range of timescales. The variability of active galactic nuclei (AGNs) in the ultraviolet and optical is usually at the few tens of per cent level over timescales of hours to weeks1. Recently, rare, more dramatic changes to the emission from accreting SMBHs have been observed, including tidal disruption events2,3,4,5, ‘changing look’ AGNs6,7,8,9 and other extreme variability objects10,11. The physics behind the ‘re-ignition’, enhancement and ‘shut-down’ of accretion onto SMBHs is not entirely understood. Here we present a rapid increase in ultraviolet–optical emission in the centre of a nearby galaxy, marking the onset of sudden increased accretion onto a SMBH. The optical spectrum of this flare, dubbed AT 2017bgt, exhibits a mix of emission features. Some are typical of luminous, unobscured AGNs, but others are likely driven by Bowen fluorescence—robustly linked here with high-velocity gas in the vicinity of the accreting SMBH. The spectral features and increased ultraviolet flux show little evolution over a period of at least 14 months. This disfavours the tidal disruption of a star as their origin, and instead suggests a longer-term event of intensified accretion. Together with two other recently reported events with similar properties, we define a new class of SMBH-related flares. This has important implications for the classification of different types of enhanced accretion onto SMBHs.

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Fig. 1: The persistence of X-ray and UV continuum, and of line emission in AT 2017bgt.

Fig. 2: Optical spectrum of AT 2017bgt compared with known unobscured AGNs.

Fig. 3: AT 2017bgt as part of a new class of SMBH-related flares in galaxy nuclei.

Fig. 4: Broad emission features near He iiλ4,686 in AT 2017bgt, and similar objects, compared with other nuclear transients.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request. All of our spectra are publicly available on the Weizmann Interactive Supernova data REPository (WISeREP)69. The data used to prepare Supplementary Fig. 1 are available from the ASAS-SN Light Curve Server (https://asas-sn.osu.edu/).

Acknowledgements

B.T. is a Zwicky Fellow. I.A. is an Einstein Fellow. E.K. is a Hubble Fellow. We thank N. Caplar, J. Guillochon, Z. Haiman, E. Lusso, and K. Schawinski for useful discussions. We thank C. Tadhunter for providing the spectrum of the F01004-2237 transient and his helpful comments. Part of this work was inspired by discussions within International Team #371, ‘Using Tidal Disruption Events to Study Super-Massive Black Holes’, hosted at the International Space Science Institute in Bern, Switzerland. We thank all the participants of the team meeting for their beneficial comments. Support for I.A. was provided by NASA through the Einstein Fellowship Program, grant PF6-170148. C.R. acknowledges support from the CONICYT + PAI Convocatoria Nacional subvencion a instalacion en la academia convocatoria a no 2017 PAI77170080. P.G.J. acknowledges support from European Research Council Consolidator Grant 647208. A. Horesh acknowledges support by the I-Core Program of the Planning and Budgeting Committee and the Israel Science Foundation. G.H., D.A.H. and C.M. acknowledge support from NSF grant AST-1313484. M.B. acknowledges support from the Black Hole Initiative at Harvard University, which is funded by a grant from the John Templeton Foundation. G.L. acknowledges support from a Herchel Smith Research Fellowship of the University of Cambridge. Ł.W., M.G. and A. Hamanowicz acknowledge Polish National Science Centre grant OPUS no 2015/17/B/ST9/03167 to Ł.W. Research by D.J.S. is supported by NSF grants AST-1412504 and AST-1517649. E.Y.H. acknowledges the support provided by the National Science Foundation under Grant No. AST-1613472 and by the Florida Space Grant Consortium. This work makes use of observations from the Las Cumbres Observatory network. This publication also makes use of data products from the Wide-field Infrared Survey Explorer. WISE and NEOWISE are funded by the National Aeronautics and Space Administration.

This work made use of data from the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by the National Aeronautics and Space Administration. We thank the NuSTAR operations, software and calibration teams for support with the execution and analysis of these observations. This research made use of the NuSTAR Data Analysis Software (NuSTARDAS) jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (USA).

We thank the Swift, NuSTAR and NICER teams for scheduling and performing the target-of-opportunity observations presented here on short notice. The LRIS spectrum presented herein was obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The observatory was made possible by the generous financial support of the W. M. Keck Foundation. We recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.

These results made use of the Discovery Channel Telescope (DCT) at Lowell Observatory. Lowell is a private, non-profit institution dedicated to astrophysical research and public appreciation of astronomy and operates the DCT in partnership with Boston University, the University of Maryland, the University of Toledo, Northern Arizona University and Yale University. The upgrade of the DeVeny optical spectrograph has been funded by a generous grant from John and Ginger Giovale.

The FLAMINGOS-2 spectrum was obtained at the Gemini Observatory under program GS-2017A-Q-33 (PI: Sand), which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the National Science Foundation on behalf of the Gemini partnership: the National Science Foundation (USA), the National Research Council (Canada), CONICYT (Chile), Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina) and Ministério da Ciência, Tecnologia e Inovação (Brazil).

Contributions

B.T. and I.A. led the data collection, analysis and interpretation, as well as the manuscript preparation. C.R. performed the analysis and modelling of archival and new X-ray data. S.T. performed the morphological and SED modelling of the host galaxy. D.S., M.B., M.H., N.K. and G.B.L. took part in obtaining and calibrating the Palomar and Keck spectra. H.N. contributed to the identification and interpretation of the Bowen fluorescence spectral features. P.G.J. and A. Horesh contributed to the interpretation of multi-wavelength data and to pursuing follow-up observations. J.E.M.-R. contributed to the analysis of optical spectra. G.H., V.H. and C.M. contributed to collecting, calibrating and analysing the Las Cumbres Observatory and Swift/UVOT data. D.A.H. helped schedule and monitor the data from the Las Cumbres Observatory. Ł.W., M.G. and A. Hamanowicz contributed to NIR line identification and provided the optical spectrum of OGLE17aaj. S.B.C. provided the the DCT spectrum. D.J.S. provided the the Gemini-South/FLAMINGOS-2 NIR spectrum. E.Y.H., M.M.P. and T.R.D. provided the Magellan/FIRE NIR spectrum. E.K. contributed to the X-ray data analysis and interpretation. K.C.G., Z.A. and R.R. contributed to the NICER data acquisition and calibration.